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This paper describes the emission reduction technologies applied to 4- and 6-cylinder engines used on Japanese market models certified as ultra-low emission vehicles (U-LEVs) in Japan. To qualify for this rigorous U-LEV certification, a vehicle must reduce hydrocarbon (HC) and nitrogen oxide (NOx) emissions by an additional 75% from the levels mandated by Japan's 2000 exhaust emission regulations. Nearly all Nissan Japanese models fitted with a gasoline engine, ranging from in-line 4-cylinder engines to V6 engines, have now been certified as U-LEVs. This has been accomplished by further improving the emission reduction technologies that were developed for the Sentra CA, which was launched in the U.S. market in 2000 as the world's first gasoline-fueled vehicle to qualify for Partial Zero Emission Vehicle (P-ZEV) credits from the California Air Resources Board.

The specific new technologies involved are as follows. The adoption of a newly developed tandem catalyst system has made it possible to obtain high conversion efficiencies for both HC and NOx emissions with a smaller catalyst capacity. A new catalyst coating has also been developed that is specifically designed to shorten catalyst light-off time. In addition, a new HC trap catalyst has been developed that attains a high HC conversion rate at low temperatures. The use of a fast-acting oxygen sensor has made it possible to reduce the engine-out HC level. On the basis of simulations and experimental results, a new exhaust manifold has been developed, along with optimizing the valve timing. As a result, by suppressing the decline in exhaust gas temperature right after engine start, it is possible to obtain high power output combined with low emission levels. Moreover, a system has also been adopted for reducing engine-out HC emissions. The application of these newly developed technologies has now made it possible to obtain U-LEV certification in Japan for nearly all Nissan passenger car models.